Press with hydraulically controlled blank holder



D. C. VERSON Feb. 1, 1949.

2,460,612 PfiESS WITH HYDRAULICALLY CONTROLLED BLANKHOLDER I Filed April 26, 1943 2 Sheets-Shet 2 Patented Feb. 1, 1949 PRESS WITH HYDRAULIC/ALLY CON- TROLLED BLANK HOLDER David C. Verson, Chicago, Ill., assignor to Verson Allsteel Press Company, Chicago, 111., a corporation of Delaware Application April 26, 1943, Serial No. 484,505

18 Claims. 1

My invention relates to presses. More particul-arly, it relates to so-called double acting presses wherein one reciprocable slide (which for convenience may be termed the work slide) carries a punch or die for performing the desired operation upon the work piece, and another slide (which for convenience may be called the blank holder slide) efiects the retention of the-work piece in position during such operations as draw- One of the objects of my invention isto provide.

an improved double acting press.

, Another object is to provide a press wherein the punch or die actuating (i. e., work) slide is of the so-called mechanical type (i. e., driven by a crank, eccentric or the like) and the work is held in place by fluid pressure. There is thus combined advantages of the mechanical press, such as speed, and also advantages of holding the work with constant and easily adjustable and flexible fluid pressure.

.Another object is toprovide a press wherein the work is retained in position by hydraulic pressure applied at a plurality of points in such manner as to compensate for variations in work thickness, distortion of the press frame which may occur under extreme forming pressures, and other conditions which might result in the work being inefiectively held.

A further object is to provide adequate movement of the blank holder slide with a small movement of hydraulic fluid.

An additional object is effectively to retain the work in place under high'clamping pressure with little power in addition to that required to do the desired work.

Another object is to provide for the more effective stripping of the punch or die from the work.

A further object is to provide a more efiective release and elevation' of the blank holder from the work.

A further object is to alTord protection to press parts against shock when the blank holder is moved from clamping position. e

Other objects and advantages will hereinafter appear. 7

The invention is shown and will be described or embodied in a press equipped with a rectangular blank holder for performing a drawing operation to produce a rectangular box-like structure. Y Y

In the drawings, c

Fig. 1 is an end elevation in partial section of an embodiment of my invention incorporated in 2 a press which, for sake of simplicity, is largely fragmentary and diagrammatic;

Fig. 2 is an enlarged plan in partial section; Fig. 3' is an enlarged vertical section of the hydraulic system for operating the work holder to clamp the work in position in the press, and

Fig. 4 is an enlarged vertical section of the hydraulic system for elevating the work holder from the work.

In generalthe press comprises a frame A, of which only enough to make clear the application of the present invention is shown, a stationary die B, a reciprocable work slide C carrying a punch or die D, a mechanical arrangement E for reciprocating the work slide, exemplified by a crank 5 and a connecting-rod 6, and a rectangular reciprocable blank holder slide F. Associated with these usual elements of a common variety of power press are the hydraulic system G for actuating. the' blank holder slide whereby the work is clamped in position to be operated upon by the punch and die, and a hydraulic system H for insuring proper stripping of the punch from the finished work, and for releasin the work and returning the blank holder to initial or work receiving position after the punch has been forced or stripped from the work. In the embodiment chosen for purposes of illustration, the work slide travels twice as far both up and down as does the blank holder slide. Thus, for example, the work slide may travel fifty inches from the top to the bottom of its stroke, whereas the vertical movement of the work holder slide is only approximately twenty-five inches. Of course, the slides are guided and laterally supported by suitable ways on the frame, but they have been omitted from the drawings in order not to complicate the disclosure with mechanism extraneous to the invention.

In the press shown, each of the two hydraulic systems acting upon the rectangular blank holder slide consists of four similar units so that only one unit of each system needs be described in detail,

Referring particularly to Fig. 3, one unit of the hydraulic system Grv for moving the blank holder slide F to and against thework piece and then causing this slide to clamp the work piece so as to retain the work in position sufficiently to enable the desired operation to be performed upon it and yet permit it properly to flow from the edges during the drawing operation performed thereon will be described in detail.

5 Blank holder slide F is mounted to reciprocate vertically in the press frame. On the down 3 stroke it presses the rim of the work piece 9 tightly but not absolutely imm-ovably against the blank seat it, which is usually a part of the die, and then continues to clamp it there during the drawing operation. It carries a tubular plunger i l which is rigidly attached thereto in any suitable manner as by. welding. Plunger ll telescopically extends into a sleeve I2 which is rigidly carried by the work slide C so as to reciprocate therewith under the drive of the crank and connecting-rod -5. Sleeve i2 is,- except for an internal packing-seat flange l3; .near.its lower end, of slightly larger internal diameter than the external diameterof plunger H. Suit: able packing i l, held in place and; compressed against flange i3 and plunger H by a cap-screw retained gland l5, provides a liquid-pressure tight fit between the plunger H and sleeve [2.) Bolted to the upper end cf sleeve i2 is a tubular cylindrical bore ofabracket- 2i whichis suitably rigidlyattached in a stationary position. on the pressfra me. Packing 22;a11d a-gland '23 serve-to provide a" fluid-pressure tightbut sliding fit between bracket 2i and plunger 2}}: The upperendcf: cylinder 21- is closed bya head" 24 secured thereto by'cap screws 25-. Head 25- has a cylindrical dome 2-6 formed integrally therewithand which houses the operating mechanism for a valve 21; A casing 23; istight-ly fitted and secured to head 2d in any appropriate manner as by welding to form an oil storage reservoir. Dome 2 .3 is qividedinto two compartments by a partition 3ilthroughwhich the stem 31' ofvalve ilgslidably fits. I The lower compartment of dome at. n onst nt gm un a i n t s voir 2 3 through ports 32' and; valve tj controls a sort 3; wh ch xten thro h head 2% 110 vented to atmosphere through ports 55; and the upper; compartment 56 above the piston 50 is permanently connected by a pipe 5"! and through a pressure control valve 58 to the air storage tanleli-i so that piston 50 is subjected always to downwardly directed air pressure the force of which depends upon the setting of control valve 58;. At} maximum this pressure, in the instance given, is 80 pounds per square inch. The relative areas of pistonBll and its stem 5! are preferably such that, for example, the 80 poundsper square inch of pressure supplied above the piston from tank; 4+ can exert a pressure-0r a fo d ommun c ti n. e w en he lowerpartrnentandthe boreoj'sleeve 211 The. upper omp r m nt. oi c me. 5' s in on an 6 111 ica on. w t th nterio q f e yoit iou h por 1am nit fits ecipro t n s env a t. WhiGhfiS attashedfiothe. p er. end f valvev em 3 L. igh si g sp in s .31 rseet va e. pwardly torc eedsositionpre rably h a or e only slightly. mo e. than. th t q ire o co nter alanc the e ght of he va ve a dl'otl er. m p t Su ahlena sina r sealing in s. hou d. he, mte nqs d. between the outer; wall: of piston h d. at. nd: he nner wall. o he pper. compartment ome .6. w reyent; eakage therebe w n iioilunde s ch pressures asmav exist. An air pressure pipe in is threaded into ad r /nine b m s the 1 12.6 ead dc ev 6 so s oi ne e air'p ssure 0f about D. p unds p r square inch from acompressedairstoragetank l'l t fhe im art d to h per ch mber f hed me above piston head 36'. This air connection is opened and closedby a solenoidvalve 42 whose function andoperaticn will'be subsequently explained; A pipe 43 screwed into an opening throughhead 214 and'a pipe 44; which is in ccmmu nication with reservoir 2 8; provide aliquid connection.between thereservoir 28' and the interior oithe piston cylinder combination 21, 28; i2 and H 1 This liquid-connection iscontrolled-by a sole ncid pressure release; valve tit whose function and operation willbe subsequently explained,

Attached tqstationary sleeve 2 by a bracket fi' is an air cylindert'lz Slidably mounted in perhaps more accurately expressed-resist a pressure of 2000 pounds per square inch exerted upwardly-against the bottom of stem 5! Under such conditions the blanleholder slide can exert; as a maximum, at" pressure of 2000 pounds per square inchupon'the work blankas will be hereinafter explained:

The closing and opening of solenoid valveAZ- is effected inthe'well known manner by an electrical circuit which includes a; switch 60-. By means of; this switch the energization, and; deenergizationofthe operating solenoid of" valve le' is broughtabout and timed in relation. to the stroke or work slide C by a cam fi'lisee Fig. 1:) c Q When switch fifiis closed-the solenoid of; valve 42" is energized and the valve is closed'. When v ve its pe ir un e press re om tank. :1

is permitted to flow into the upper compartment of dome 26 and exerta downward pressureupon piston; iifito open valve 2?: V Camel is drivenin some suitabl m er suc for. xamp e. s, a cn -tone ratio hain and: sprocket. heel. on. nection from the shaft oficranlgj In like mane H p ni and esingofthe li uid. elief valv 45: efie t by. arach d the nersi ation and 'd'e-energizati'on of wh ch. is, brought, about d me y a. switch, 63. e ed: upon. by. a. a r driv n from. he shatter. crank 5. Itis. of' course, to be understoodithatwhere the hy: a lic, stemq or. denresing, he..b1ank holde s. a he. ase. of th pr ss shown, mposed. f; umber. o uni s. beisglen id w c s: all units needs be connected in multiple to only ne. S Q wit hes andv Q3. 0.. hat the units f. esr em wi t efi e e in un son.

The pressure efi'ective areas of tubular plungers. l l nd. 20 w th respect e ach ther and! toth e nal r ss. sect nal ar as Qf. s eeve l2. and the bore ofbracket 2lare so related that whfen, the internal chamber termed bythem is filled i iq i (e.. a, o l nd plune rs U; will move relative to eachgother there is a. small; volume differential between their respective: displacements. The difierentialshould be suchthat through a given relative movement plunger, M will displace agreater volume offliq'uicl than plunger; 20- wherebya; pressure will, be built up to--actagainstthe stem 51 of piston Elfin opp 0- sition tothe airpressure existing above piston, 50. For example, this diiferential may be su ch that for every inch of travel, plunger H will displaceapproximatelyfourcubic inches more liquid-than will=plunger20 2 Referring now to Fig. 4, one unit of the companion hydraulic system H for insuring retention of the blank holder in blank clamping position untilthe punch has been stripped from the work and for thereafter elevating the blank holder to condition the press for the reception of another work piece will now be described. Only one needs be described because, although the press is shown equipped with four, all are identical. The blank holder F carries rigidly attached thereto a pair of parallel oppositely opening or acting hydraulic cylinders 65 and 66, the latter being attached to the slide F through a bracket 61. Slidably mounted in the cylinders are pistons 68 and 69, respectively. Pistons 66 and 69 are rigidly attached to work slide C. Thus, whenever there is relative movement between the work and blank holder slides, one of the pistons 6869 is moved further into its cylinder, while the other piston is retracted in its cylinder, which piston is going in and which is going out depending upon thedirection of relative movement between the work and blank holder slides. Suitable packing and packing glands I0 and" serve to provide pressure-tight but sliding fits between pistons 68 and 69 and their respective cylinders 65 and 66. A pipe 15 in which is interposed a solenoid valve 16 serves as a liquid connection between'the chambers of cylinders 65 and 66, such connection being opened and closed by the de-energization and energization, respectively, of its solenoid winding A branch pipel'i leads to the bore 18in the base of a shock absorber cylinder 19. A piston 80 slidably fits the upper cylindrical chamber of cylinder 19, suitable rings serving to insure a pressure-tight fit therebetween. The stem 8| of piston 66 fits the bore 18 and is acted upon from below by the liquid supplied through branch pipe 11. Air pressure is applied through pipe 82 from air pressure tank 4| to chamber in cylinder 19 above piston 86 so that the piston is always pressed downwardly by a constant air pressure that may, if desired, be varied by a pressure control valve 83. A small liquid pipe 85 leads from the space below piston stem 8| to a pressure actuated safety switch 86. This safety switch may, in a well known manner, effect the actuation of a solenoid controlling the operation of the entire press either, through a clutch or a switch in the press-motor circuit. The hydraulic system, including the cylinders 65 and 66 and the bore of the shock absorber 19 below the piston stem 8| is filled also with oil. As long as solenoid valve 16 is opened, this oil may circulate freely between cylinders 65 and 66 through pipe 15; then pipe 68 may freely move further into its cylinder 65 and at the same timeplunger 69 may move freely out of cylinder 66 and vice versa. On the other hand, when valve 16 is closed and there can be no free transfer of oil. between cylinders 65 and 66, the work slide C and the blank holder slide F are in effect interlocked so that they move together. If, however, work slideC is moving upwardly and blank holder slide F is stationary when valve I6 is closed, there can still be some slight further movement of plunger 69 into cylinder 66, thereby permitting the continuance, of some degree of relative movement between the work and blank holder slides, because such movement will build up a liquid pressure in branch pipe 11, and the chamber below the stem 8! of piston 80, opposing the air pressure against the top of piston 86. Relative movement between the work and blank holder slides will thus continue until the liquid pressure built up by the continuing movement of piston 69 into cylinder 66 balances the eiTect of the air pressure above piston in the safety cylinder 19. A cushion is thus provided between the work slide C and the blank holder slide'F for a purpose to be later explained. Solenoid valve 16 is controlled by a switch which, in a manner similar to switches 60 and 63,. 'is controlled by a cam 9| driven from crank shaft 5. When switch 96 is closed, solenoid switch 16 is closed'to, close the liquidv connec tion between cylinders 65 and 66. I

The complete operation of the press will now be explained. Assuming that the crank shaft is at the top of its stroke, the work slide C, the blank holder F and the telescopic cylinderplunger system ||-|220-2| will be in the respective positions indicated by the dotted lines of Fig. 3-work slide C say 50 inches above its lowermost position and blank holder slide F approximately 25 inches above its lowermost. position. During the subsequent first quarter of a revolution of the cranki. e., While it is moving clockwise as indicated by the arrow-from its top position to the position shown in Fig. 1', the work slide C moves down toward the work half of its travel. Also the blank holder slide F is carried with the work slide to the full extent of the downward travel of the blank holder slide and against the work piece- During this first quarter of a revolution of the crank, the work and blank holder slides are caused to travel together because from the beginning to the end of this quarter revolution the switch 90 controlling solenoid valve 16 hasbeen closed and retained closed. Therefore, solenoid valve 16 is closed and the flow of oil between cylinders 65 and 66 is prevented. Thereafter the work and blank holder slides have been effectively interlocked and both have traveled downwardly together. However, a very short interval before the blank holder strikes the work piece, cam 9| may open switch 96 so as to de-energize solenoid Valve 16 and permit it to open. During this first quarter cycle, the cam 6| is always in such relation to switch 66 that this switch remains open. Solenoid valve 42 is therefore tie-energized and open and oil from the reservoir 28 flows into the hydraulic plunger-cylinder system |||2--26-2| in sufficient amount to compensate for the withdrawal of plunger 26 from the bore of bracket 2!. Thus this telescopic hydraulic plunger system is retained full of oil. Also during this first quarter of crank shaft rotation, the cam 64 holds the contacts of switch 63 closed so that the solel'lOld of liquid relief valve 45 is energized and this valve is held closed so that no oil flows between reservoir 28 and the bore of bracket 2| through the pipes 43 and 44.

When the crank shaft has completed its'first quarter phase of rotation and enters upon its second phase, the cam 6| closes switch '60 with the result that valve 42 is energized and closes so that air pressure from tank 4| is removed from wardly traveling sleeve I2. The volumediiler entialzr between the; positive displacement caused:-

byrsleevei2; traveling; down .overszplungert tr, and? the: negative displacement caused;- by plunger 21s.: moving outiof. the bore of bracket 2;i'.',.b11i1ds up:

a, pressure below.- valite Z]: toclose it and below;

the-stem; 5i of; piston; fill, tending. to. elevate 1 that piston against the air pressure from. tank: i=1; Thus; the. pressure. exerted": downwardly: against the-.worhpieceby'blank. holder. slide F builds up until: there is: a balance between the internal* pressure irLthecylinder-plunger system te 2 file-2:1; and. the. downward air pressure exertedagainst piston 58, in. the illustrated example be ing 20601poundsper squareinch. In this manner, by the time the punch carried by work slide C has started to perform the desired operationuponzithe" workpiece, thatwork'pieoe is tightly clampedzby. the blank holder-slide-and continues so tmbe clamped. throughout. the drawing opera tion. which is. performed by the punch during thesecond; quarter phase ofthe rotationof its Iii tion,. i'..e.,..as-. thework slide C rises after completion of theop'eration of 'the worli piece, cam V 6.1 continues. to hold switch 66 closedso'that valve; 42: remains closed and in consequence,

valve, 211 is: held. closed by the'pressure built up and?v remaining. below: it; Thus, although the workfslide rises, the .blank holder slide ishelddown to: retain the work. piece in position and proper-stripping of the punch from the work is insured. To permit this relative movement be 1 tween, the work.- and blankholderi slides, switch 9.0 remains openitokeep. valve 16? open and therepermit a free;exchange of oil between cylinders 65-.and 66;

As the fourth and. last quarter of crank rotation commences,.va1ve;45 is opened bycarn 54, permitting switch G-Stoopen. The opening of valveda permits'oil' to. flow from the until then to reservoir 28,. effecting: a: release of. the liquid pressurebelowvalve 2.1.v At-the'same time cam 6| opens switch 6U? which energizes the solenoid switch-4?.- to, permit pressureyfrom tank 4 to act upomthe-ba-ck of piston. 36. This air pressure tending to open valve 21, couplediwiththe: relief ofopposing. pressure effected by the; opening of valve.. 4.5, enables valve-21: to-open. Cam 9i has.

closed. switch 90. and holds it closed. during the lastiqnarter. phase so that valve 16-is energized andlclosed and flow-:of oilbetweencylinders 55- workpiece by the, blank holder slide, enables the blank holder to accommodate itself'tosomevariations in the thickness of thework i'nadiiferent places: arrdzstill: effect. proper distribution; of; clamping" pressure, The resiliencyof. the; air column in cylinder IQ-cushions.- the= startlng'rofi the relative movements between they work: and:

blankholder slideszand so relieves. themechanisms.

from undue shock; In the event: the pressurezirr cylinder 56 tends to become unduly high,..it oper ates-safety switch as to stop the: press. 7

Having thus explained. the nature ofmy in; vention and described a typical. embodiment thereof, what I claim and desireto se'cureby United States LettersPatent is asfollowsr 1. A punch press having a 'crankdriven work slide and a blank holder slide, characterized by an hydraulic system for' interlocking the two slides, said-system including apairof reverse'l'y acting cylinderplunger sets connectedtb the twoslides, sothat as one-set expands, the other-set contracts when relative movement occurs be tween thetwo'slides, a fluid connection between the cylinder-plunger sets so that liquid" may transfer from the contracting tothe expanding set to permit relative movement therebetween,.; a solenoid valve in" the fluid connection topro-- vent transferring of liquidand thereby-interlock the two slides, a circuit including a switch for controlling the valve; and :a, cam driven by the crank shaft for timing the opening and closing ofsaid switch. 7

2'; A punch press having a crank operatedworl' slide anda blank holder slide; characterized by an hydraulic pressure system actuated by thework slide for actuating the blank holder-slide to cause it toclamp the-work; anhydraulic sys term for interlocking" the: two slides to cause-themto 'movetogether; solenoid operated valves-Tori controlling the two hydraulic systems, circuits including switches for controlling' the' solenoid valves,,and'v cams operated from the work slide crank to openand close thecontacts.

3. A punch press having a mechanically driven work slide in combination with" a blank holder slide, an hydraulic system for-pressingthe blank holder slide in workclamping-position, theprese sure in thehydraulic system being built up by; themovement of the slide towardthe work after the blank holder has reached clampin'g position; and another hydraulic. system for raising" the blank holder slide out of. work clamping ppsition;. the second mentionedhydraulic system interlocks in the blankihold'er slide and the workslide dur ing that portion of an operation cyclewhen the blank holder slide, is. being moved toward" and away fromwork clamping position.

4. Apunchpress having a crank driven work slide and. a blank holder slide, characterized by? an hydraulic system for interlocking, the. two slides,.said system including a pair. of reveisely acting cylinderepiunger sets connected. tothetwo: slides, sothat. as oneset" expands, the otherrset" contracts when relative movementoccurs'between' the two slides, a. fluid" connection between the" cylinder-plm1ger'setsso that liquid may transfer from the contracting to the, eiipandingset to permit relative movement therebetween; a valve in' the fluid connection to prevent transferring of liquid" and thereby interlock' the two' slides, and means actuated by the crank shaft for'timing the opening and closing of said valve. 7

. 5; A punch press having a mechanically "driven' workslide" in combination with a blank holder slide, 'an hydraulic' system" for interlocking the work slide an'd the-blank holder slidewhen the blank holder-slide is moving towardarrd away from work. clamping position-a valve for controlling the flow ofifiuid in the hydraulic system, and timing means actuated by the mechanical driveand operating said valve to render the hydraulic system effective during a portion of a cycle to lock the blank holder slide to the work slide for movement to and from work clamping position, and during another portion of the cycle to interlock said slides to permit relative movement between said slides.

. 6. A punch press having a mechanically driven work slide in combination with a blank holder slide, means connecting the work slide and the blank holder slide andpermitting relative movement therebetween, and hydraulic system actuated by the work slide for actuating the blank holder slide to cause it to clamp the work, a valvecontrolling the, creation of pressure in the hydraulic system, another hydraulic system forming a portion of the means connecting the work slide and the blank holder slide, a valve inthe second mentioned hydraulic system for preventing flow of fluid therethrough to interlock the work slide and the blank holder slide, and timing means operated by the mechanical drive for each s valve to open and close the valves in. unison with the movement of the work slide.

'7. A punch press having a mechanically driven work slide in combination with a blank holder slide, an hydraulic system for pressing the blank holder slide in work clamping position, another hydraulic system adapted to connect the blank holder slide with the work slide, a valve for controlling the flow of fluid in said second hydraulic system, :andtimin means operable in unison with the reciprocation of the work slide to operate said'valve to render said second hydraulic system effective to interlock the blank holder slide and the work slide to cause the latter to advance and withdraw the former to and from the work clamping position, said valve being operable by said .timing means to render said second hydraulic system inoperative so as to break the interlock when the' blank holder slide is in the work clamping position to permit relative movement between the blank holder slide and the work slide.

8. A punch press having a mechanically driven work slide in combination with a blank holder slide, an hydraulic system for pressing the blank holder slide in work clamping position, another hydraulic system adapted to connect the blank holder slide with the work slide, a valve for controlling the flow of fluid in said second hydraulic system, timing means operable in unison with the reciprocation of the work slide to operate said valve to render said second hydraulic sys t'ein effective to'interlo'ck the blank holder slide and the work slide to cause, the latter to advance and withdraw the former to and from the work clamping position, said valve being operable by said timing means to render said second hydraulic system inoperative so as to break the interlock when the blank holder slide is in the work clamping position to permit relative movement between the blank holder slide and the work slide, and means including timing means operable in unison with the reciprocation of the work slide to relieve the pressure in the first hydraulic system whilethe blank holderslide is being withdrawn from the work clampin position.

9. In a double action press, a mechanically reciprocable die slide mechanism, a reciprocable work clamping mechanism, hydraulic mechanism operatively disposed between and adapted to inlo terlock the work clamping mechanism immovable with respect to the die slide whereby one a of said slides moves the other, a control system for the hydraulic mechanism operable upon predetermined reciprocation of one of said slides to render said hydraulic mechanism operative and inoperative for connecting and disconnecting the work clamping mechanism from the die slide, and means independent of said interlocking mechanism interconnecting the die slide and the work clamping mechanism and the frame of the press for exerting pressure from the die slide upon the work clamping slide when the die slide is operatively disconnected from the work clamping slide by said control system.

10. In a press, a mechanically reciprocable slide, a work holding slide mechanism, hydraulic mechanism connecting the work holding 'mechanism to the reciprocable slide whereby the two slides are adapted to move in unison during a part of the cycle of an operation thereof, a control system for the hydraulic mechanism operable to permit movement of one of the slides while movement of the other slide is arrested, a differential duplex piston and cylinder mechanism interposed between the slides for exerting hydraulic pressure upon the arrested work holding mechanism and during movement of the other slide, and means associated with the piston and cylinder mechanism for maintaining the pressure therein uniform during the arrested period of said holding slid mechanism.

11. In a press, a mechanically reciprocable slide, a second slide mechanism, a controllable locking mechanism operatively disposed between and connecting the second slide mechanism to the reciprocable slide whereby the two slides are adapted to move in'unison during a part of a cycle of an operation thereof, a control system for the locking mechanism operable to permit movement of one of the slides while movement of the other slide is arrested, a piston and cylinder mechanism interposed between the slides for exerting hydraulic pressure upon the arrested secand slide mechanism and during movement of the other slide and whereby the pressure is generated by such relative movement, and means associated with the piston and cylinder mechanism for maintaining the pressure generated therein constant during relative movement between the two slide mechanism.

12. In a double action press the combination of a die slide, a clamping slide, a hydraulic locking mechanism'connecting the clamping slide to the die slide, valve means for controlling said hydrauliolocking mechanism whereby the relative vertical position of the die slide and the clamping slide can be varied as desired during the raising and lowering of said slides, a shock absorber means associated with the hydraulic locking mechanism-for protecting the clamping slide when downward movement thereof is arrested by a work blank, a hydraulic piston and cylinder mechanism mechanically connected to the die slide and the clamping slide and to the frame of the press for generating pressure eX- ertable upon the clamping slide during relative movement of the slides, means for maintaining such, generated pressure uniform, and control meansfor breaking the pressure in the piston and cylinder mechanism prior to initiation of an upward movement of the clamping slide.

13. In a press the combination of a mechanically actuated slide, a second slide raised and. lowered by the first named slide, means for recip- 'r'o'cating oneof said slides, an intermittently 0D- erable locking mechanism connecting the two slides,-'a control mechanism operable :upon r'e-I ciprocation of onelof said slides for effectingdisconnection of the slides during part .of the cycle of action :of the-mechanically actuated slide, a

lduplexhydraulic piston and cylinder differential mechanism.interconnected between the twoslides including a piston andcylinder carried :by the mechanicall actuated slide for generatin pressure uponthe second slide during 'relativermovemerit between :the slides,-means for maintaining such generated pressurelat a predetermined constant-value, and a pressure breaker anechanism operable prior to the ilockingaction of said lock- :ing mechanism. r

14. "In-la doubleaction press, a dieslide; -a work clamping slide, saidslides being movable-relative to one another, means for reciprocating one of said slides, means for interlocking said slides in a'pluralitylof selected, relative positions whereby said slides are movable as a unit in opposite/di- -rections during predeterm ned movements 101 said slide, said interlocking means including, two fluid containing cylinders each having apiston recip- --rocable therein, said cylinders and theiricooperating:pistons'beingscarried-by opposite ofsaid slides wherebythe cylinders and pistons are movedrelative to one another when saidslides are moved relative to :oneanothensaid cylindersand :pistons being arranged so that when one piston is withdrawn relative to its cooperating cylinder the :other of :said pistons is telescoped :relative to its cooperating cylinder, conduit means interconnecting said cylinders'zwhereby'fluid is transferable from one rcylinder to the other when said slides-are imoved'relative to one another, and a valve 'for controlling the :floW .of fluid through said conduit means.

:15. In a double action press, 'a mechanically reciprocable slide 'tmechanism, 1a u'eciprocabl'e work clamping mechanism, a hydraulic mechaz'ism adapted to connect the work clamping mechanism to the die slide during part of the cycle of operation of saidslideaa control system ior' the hydraulic mechanism operable upon reciprocation or one of said slides to hydraulically disconnect the work clamping mechanism from the-die slide, a duplex differential piston and cylinder mechanism interconnecting the die slide 7 and the work clamping mechanism and the frame of the' press forexertingpressure-upon the work clamping slide when the die slide is operative'h disconnected from work clamping slide by said control system, and means operable during said pressure exertion for maintaining a constant volume, uniform pressure condition in said piston and cylinder mechanism. 7 18. Ina sheet metal drawing press; the combination of a press frame, -a drawing die slide, a :blank holder slide, :mechanical means for reciprocating "the die slide, pressure generating hy- :draulic mechanisms operative'l-y disposed between the two slides and located at opposit sides of the die slide and operable to generate pressure "when relative movement "between the two slides takes place, a hydraulic locking device operatively connected to the two slides whereby the slides be moved in unison and also may be moved relative to each other, valve means controlling the locking device-whereby the die slide can raise and lower the'blank holder slide during part of the :cycle of operation of the die slide, means operated in synchronism with one of said slides adapted to operate said valve means to hydraulically disconnect the 'two slides when .the blank holder'is lowered into conta-ct'with-a work blank, a constant pressure constant volume maintainmg means associated with each of the pressure generating hydraulic mechanisms ior maintainin a predetermined uniform pressure upon the blanlcholder slide during both a downward and upward movementoi the die slide, a fluidreservoir associated with each of the pressure'generating hydraulic mechanisms for receiving fluid from and supplying fluid to said mechanisms when the two slides are being moved in unison, valve means controlling said reservoir supply and controllable in-synchronism with one of said slides, and a pressure breaking means associated with the reservoir'supply and the pressure generating hydraulic mechanisms for breaking *the pressure upon the blank holder slide before -the die slide i hydraulicallylockedto the blank holder slide during an upward movement of the die slide. J-

17. In a sheet metal. drawing press, the combination of a press frame, a drawingdie slide, a blank holder slide, mechanical means for reciprocating the die slide, pressure generating hydraulic mechanisms operativelydisposed between the two slides andlocated at opposite sides of ie die slide, a hydraulic locking device operatively connected .to the two slides, valve means controlling the locking device whereby the die slide can raise and lower the blank holder slide, a shock absorber means associated with the lockdevice for absorbing shock when movement of the'blank holder slide is being arrested, means operated insynchronism with one of said slides adapted to operate said valve means to hydraulically disconnect the two slides when the blank holder is :loweredinto contact witha work blank, a constant pressure constant volume maintaining means associated with the pressure generating hydraulic mechanisms for maintaining said predetermined pressure upon the blank holder slide during part of a downward and upward movement of the die slide, a fluid reservoir associated with the pressure generating hydraulic mechanisms when the two slides are being moved in unison, valve means controlling said reservoir supply and operable in synchronism with .oneof said slides, and a pressure breaking means associated with the reservoir supply and the pressure generating hydraulic mechanisms for breaking the pressure upon the blank holder slide before the die slide is hydraulically locked to the blank holder slide during'an upward movement of the die slide;

fluid under low pressure for the three part plunger cylinder combination, a valve controlling fiuid flow between the source and the three part phmgercylinder combination and arranged so as to be held in closed position by pressure built up in the three part plunger-cylinder combination, valve operating means, timing means controlling the valve operating means and timed to the press cycle to render said valve operating means operative to open the valve during that 'part of the press cycle when the blank holder slide is moved toward and away from the work and to render said valve operating means inoperative during the blank holding part of the cycle so that the valve is held in said closed position, a pressure limiting control system for the three part plunger-cylinder combination including a pneumatic cylinder having a pneumatic pressure biased plunger therein and an hydraulic cylinder having an hydraulic pressure biased plunger therein connected to the pneumatic plunger, the hydraulic cylinder being in constant communication with the three part plunger-cylinder combination so that the pressure acting against the hydraulic plunger is always the same as the hydraulic pressure in the three part plunger-cylinder combination and is connected to uphold the pressure in the pneumatic cylinder,

whereby the pressure in the three part plunger--v cylinder combination'is limited by the pressure in the pneumatic cylinder and the pressure exerted by the blank holder slide is controlled by said pneumatic pressure, and a normally closed pressure relief valve positioned between the fluid source and the three part plunger-cylinder combination and timed to the press cycle to open at the end of a blank holding operation thereby to REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 'Number Name Date 662,515 Yale Nov. 27, 1900 1,180,626 Walker Apr. 25, 1916 1,433,116 Beyer Oct. 24, 1922 1,774,614 Rode Sept. 2, 1930 1,844,250 Hawkins Feb. 9, 1932 2,113,110 Ernst Apr. 5, 1938 2,217,172 Laussucq Oct. 8, 1940 2,252,881 Ernst Aug. 19, 1941 2,294,451 Ernst Sept. 1, 1942 2,305,336 Williamson Dec. 15, 1942 2,325,146 Muller July 27, 194? FOREIGN PATENTS Number Country Date 524,701 Germany 1931 

